1. Field of the Invention
The present invention relates to an MIS (Metal Oxide Insulator) transistor having an MIS structure in which an insulating film and a metal electrode are formed on the surface of a semiconductor.
2. Related Background Art
MIS field effect transistors formed on thin film insulating substrates have recently attracted attention as high-speed semiconductor devices. The structure of such MIS semiconductor devices is basically the same as that of an MIS field effect transistor formed on a bulk substrate. FIG. 1A is a schematic sectional view showing an example of such MIS field effect transistors. In FIG. 1A, reference numeral 151 denotes a ground silicon oxide film; reference numeral 152, a semiconductor layer; reference numeral 153, a thermal oxidation film; reference numeral 154, a polycrystalline silicon film; reference numeral 156, a sulfur nitride glass layer; reference numeral 157, an interlayer insualting film; reference numeral 158, wiring electrodes; and reference numeral 159, a protective film. FIG. 1B is a drawing showing the energy band of the MIS field effect transistor shown in FIG. 1A in the channel direction in thermal equilibrium, and FIG. 1C is a drawing showing the energy band of the same transistor when a drain voltage is applied thereto. In this MIS field effect transistor, the source and drain portions are doped with boron, phosphorous or arsenic. As shown in FIG. 1B, the energy band gap Eg.sub.1 of the source and drain portions is the same as the energy band gap Eg.sub.2 of the channel portion.
As described above, since the MIS field effect transistor has the channel portion comprising the thin film semiconductor layer formed on the insulating substrate, non-transport carriers (holes in the case of N-MOS) which are produced by impact ionization at the drain edge are accumulated in the channel portion. As a result, the electrode potential in the channel portion is decreased, thereby causing the problem that a kink phenomenon occurs, and the source-drain endurance voltage is decreased. Namely, since the energy band gap Eg.sub.1 of the source and drain portions is equal to the energy band gap Eg.sub.2 of the channel portion, charge is concentrated at the drain edge, and many parts of electrons 113 and holes 114 are thus produced by the impact ionization, as shown in FIG. 1C. Although the electrons 113 produced are discharged to the drain electrode 112, the holes 114 flow in the channel portion 115. In MOS transistors comprising a thin film silicon oxide substrate, an electrode for fixing the potential of the substrate is sometime not provided in order to obtain the thin film effect of improving the mobility or the like. In this case, the holes are inhibited from flowing in the source by the potential wall and accumulated in the channel portion without being discharged, resulting in a decrease in the electron potential, as shown in FIG. 1C. The steady state is thus maintained by decreasing the height of the potential barrier between the source and channel portions to a value smaller than the intrinsic height so as to partially discharge the holes. This state corresponds to the state wherein a positive voltage is applied to the substrate, i.e., the state wherein the Vth value is shifted to the negative direction in a N-MOS transistor. This is generally called "substrate floating effect". This phenomenon becomes significant when a drain voltage is applied and causes the fault that the drain current Id is abruptly increased when the drain voltage is increased to a certain value, as shown in FIG. 2. For example, in a field effect transistor having a gate length of 1 .mu.m, the drain current is abruptly increased at a drain voltage of about 5 volt. Even if a substrate electrode is provided for removing this fault, since the semiconductor layer has a very small thickness and a low efficiency of hole discharge, although the endurance voltage between the source and drain is slightly increased, the above problem is not solved.
An LDD (Light Doped-Drain) structure is employed as a measure to solve the above problem. However, this structure is designed for relieving the electrical field at the drain edge so as to inhibit electron-hole pairs from being generated by the impact ionization. Since most current measures to solve the problems are designed for relieving the electrical field at the drain edge, conventional structures for relieving the electrical field including the LDD structure and the like slightly increased the endurance voltage between the source and drain by about 2 volt.